Formation of Y-Ti-O nanoclusters in nanostructured ferritic alloys: A first-principles study

Abstract

Density-functional theory ͑DFT͒ calculations have been performed to study the atomic scale energies, structures, and formation mechanisms of dissolved Y, Ti, and O solutes and small Y-Ti-O nanoclusters ͑NCs͒ in a bcc Fe lattice. Key results include the following observations. The Y and O are dissolved during mechanical alloying of Y 2 O 3 with metal powders by ball milling, which provides the large requisite solution energy of about 4 eV per yttria atom. The solution energies of substitutional Y as well as interstitial O, which are referenced to elemental Y and solid FeO, respectively, are also high. Bound O-O, Y-O, and Ti-O pairs decrease the system energy relative to the isolated solutes and constitute the basic building blocks for NCs. The lowest energy configuration for a Y 2 TiO 3 NC is 5.11 eV less than the total energy of the dissolved solutes. Our DFT calculations show that Y-Ti-O NC formation can take place without the energetic assistance of pre-existing vacancies. This conclusion is significant since excess vacancies are not a persistent thermodynamic-energetic constituent of the Fe-Y-Ti-O system and will quickly annihilate at dislocations during high-temperature powder consolidation.

First-principles calculation of defect free energies: General aspects illustrated in the case of bcc Fe

“…Regarding the fact that that in Table II certain values of ( ) f E X are negative we refer to the corresponding results and their discussion in the literature. 32,40,41,44,45,47,49,66,67 In Table II …”

An in situ powder neutron diffraction study of nano-precipitate formation during processing of oxide-dispersion-strengthened ferritic steels

“…Atom probe studies (the only true atomic resolution microscopy) have recently confirmed the formation of a relatively homogeneous increasing interfacial energy as milling progresses [3]. Kinetically, the dissolution into the ferrite is facilitated by small crystallite sizes and a high vacancy and dislocation density [20,21]. 2) The diffraction peaks of ferrite from as-milled powder were significantly broadened due to decreasing crystallite size and increasing lattice strain, and this makes any underlying peak shifts very difficult to identify [19].…”

A first-principles study of helium storage in oxides and at oxide–iron interfaces

“…15 In this context, an oxide "particle" can be as small as a fewÅngströms, correponding to just a couple of atoms. 10,12 While size of these "nanofeatures" 3 can be below the resolution obtainable in transmission electron microscopy, they nonetheless contribute to He sequestration due to their sheer number and very large effcetive interface area.…”

“…7-9). They are characterized by a fine distribution of a) Electronic mail: erhart@chalmers.se nanometer-sized oxide particles [10][11][12] that act as obstacles for dislocation motion and are metastable up to very high temperatures. The oxide particles and even more so the oxide-matrix interfaces are expected to act as sinks for He interstitials.…”